Condition responsive apparatus



Jan. 16, 1962 R. P. KELLER EVAL CONDITION RESPONSIVE APPARATUS Filed June 25, 1959 RELATIVE RESISTANCE TEMPERATURE INVENTORS' ROGER P. KELLER JAMES J. RENIER ATTORNEY Uni-ted States Patent ware Filed June 25, 1959, Ser. No. 822,790 3 Claims. (Cl. 338-28) The present invention relates to improved thermally responsive resistors and more particularly to a thermally responsive resistor employing a fusible salt or electrolyte having the inherent characteristic of undergoing a change in resistivity at substantially the temperature at which salt or electrolyte undergoes the change from the solid to the liquid state. Specifically, the invention relates to resistors of this type which are provided with electrodes prepared from a substance having a non-conductive corrosive or corrosion product and these devices accordingly do not exhibit an increase in conductivity in relation to time of use. For example, aluminum in the presence of an oxidative melt forms an oxide product which is substantially non-conductive. Materials whichI exhibit this characteristic and which are suitable in connection with devices of this type are nickel, aluminum, and magnesium. It should be pointed out that the nickel is suitable in these applications provided that the operating temperature range is not above the temperature where Ni2O3 is unstable and also provided that the melt does not include the presence of a lithium ion.

According to the present state of the art, fusible salt "resistors have not been particularly widely used for certain applications because of their inherent characteristic of being fail-unsafe for series applications. This is particularly true in flame sensing applications and the like, or in other applications where the condition to be sensed is one which renders the unit conductive, and continued use of such a unit tends to continually increase the conductivity of the ll or electrolyte regardless of the physical state thereof. The device of the present invention, on the other hand, would be fail-safe in such an application inasmuch as the resistivity does not increase with use, the corrosion product or oxide product of the electrodes being substantially non-conductive.

It is therefore an object of the present invention to provide improved thermally responsive resistors which are rendered fail-safe for ame-sensing applications where, because of the inherent nature ofthe electrode material, the corrosion product of the electrodes is substantially non-conductive.

It is a further object of the present invention to provide an improved thermally responsive resistor which includes an inner and an outer electrode, the electrode being prepared from a material selected from the class consisting of nickel, aluminum, and magnesium, the corrosion product of these materials being substantially non-conductive.

Further objects of the present invention will become apparent to those skilled in the art upon a study of the following specification, appended claims, and accompanying drawing wherein:

FIGURE 1 is a horizontal sectional view taken along the axis of an improved fusible salt resistor prepared in accordance with the present invention;

FIGURE 2 is a transverse sectional view taken along the line and in the direction of the arrows 2-2 of FIG- URE l showing the transverse cross-section of an apparatus prepared in accordance with the present invention, and

FIGURE 3 is a typical plot of relative resistance versus temperature for typical thermally responsive devices prepared in accordance with the present invention.

According to the preferred modiication of the present ICC invention, a fusible resistor generally vdesignated 10 is provided with an outer cylindrical or sleeve electrode 11 and a concentrically arranged inner electrode 12. While the particular conguration of the electrode pair is not critical for operation of this device, it is essential that the distance between the electrode members be substantially constant from one point in the member to another. The electrode members 11 and 12 are further prepared from materials selected from the class consisting of nickel, aluminum, and magnesium, each of these materials having the characteristic of having a non-conductive oxide or corrosion product. The annular area or space between the electrodes 11 and 12 is filled with a composition designated 13, this composition including preferably a fusible salt or electrolyte, together with an inert iiller material such as alumina or the like. The inert substance is included in the lill in order to avoid hot spots or self-heating when it is desired that the fusible resistor act as a thermal switch. While the particular electrolyte material in the till is generally not critical to the operation of the device, many salts or electrolytes being suitable, the key or essential features being that the material is a good conductor of electricity when heated to the fusion temperature,is substantially non-conductive or dielectric in solid form, and iscompatible with the electrode material at fusion temperatures. One material which has been found particularly suitable in devices of this sort when aluminum or magnesium is employed as the electrode metal is lithium chloride-potassium chloride eutectic. Nickel is adaptable for use with sodium chloride, potassium chloride or mixtures thereof. The salt/support ratio should generally be less than l/ 1, preferably in the range of l/3 by weight when a combination of lithium chloride-potassium chloride eutectic and alumina is employed. In order to seal the ends, discs 14 and 15 are provided at the terminal ends of the body. These may be made of alumina or other suitable materials, and are set in place and retained therein by suitable means. For example, when alumina is used as the material for the discs 14 and 15, a suitable sealing substance is glass frit. Of course, other suitable materials may be utilized for the retaining members at the ends of the resistor element.

For dame-sensing applications, the preferred electrode material is aluminum, this being due to its ability to tolcrate relatively corrosive atmospheres. It is adaptable to flame-sensing applications, even though its softening temperature is in the range of 650 C. The high thermal conductivity of aluminum will tend to prevent heating of the device generally to temperatures in that range even though a local portion of the unit may be actually in substantial physical contact with the flame. Magnesium, of course, due to its combustibility characteristic at temperatures in the range of 600 C., is not particularly suited for pilot-flame detecting applications. Nickel may also be used at higher temperatures in devices employed for this purpose, aluminum being preferred however.

For lifetime considerations, it is further desirable that the salt or electrolyte, when placed in the resistor element, be substantially anhydrous in nature.

The physical size of a device of this sort is obviously dependent upon the use or application to which it is being put. Accordingly, those skilled in the art can readily ascertain the optimum range of sizes for the particular end use to which the unit is being put. For flame-sensing applications, an aluminum shell with an aluminum wire arranged therein, the outer shell having a thickness of 0.038 inch, an I.D. of 0.250 inch, and a length of about 1.75 inches. The central electrode, in these applications is suitably of the order of 60 mils. Of course, for this application, a nickel electroded device may be employed having substantially similar dimensions.

The graph of FIGURE 3 illustrates a typical response curve for devices of this type. It is seen that the relative resistance of the unit is quite high at temperatures below the fusion point of the salt fill. As the fusion point is approached, the resistance of the device drops sharply, and as the fusion point ,is substantially reached, approximately 80% of the total possible resistance drop has been experienced in the unit.

Of course, it will be obvious to those skilled in the art that the particular salt used for a particular application will be selected according to the temperature at which the sharp drop in resistance is desired. In addition, the particular circuitry arrangement in which the device is to be included, will also dictate to some degree the relative dimensions and the like of the completed unit.

It will be understood that the specific arrangements of the device as set forth herein are given for purposes of illustration only and are not to be construed as otherwise limiting the scope of the present invention.

I claim as my invention:

1. A thermally responsive resistor comprising an inner and outer relatively spaced electrode pair, and deiining an annular area of substantially uniform thickness therebetween, the annular area being at least partially illed with lithium chloride-potassium chloride eutectic, so as to deline a conductive path between said electrodes, said resistor element being characterized in that the electrode members consist essentially of a metal from the group consisting of aluminum and magnesium.

2. A thermally responsive resistor comprising an outer metallic conductive member spaced from and enclosing an inner metallic member and deiining an annular area of substantially uniform thickness therebetween, the annular area being at least partially filled with an electrolyte in the form of a salt fusible at a temperature lower than the melting point of said metallic members, said salt being normally in a solid state and nonconductive but being electrically conductive when fused, so as to deiine a conductive path therebetween, said salt being in electrical contact with the metal of said metallic members, said metallic members being made solely of metal selected from the group consisting of nickel, magnesium, and aluminum, whose oxides are nonconductive, so that the effect of continued oxidation of said metallic conductive members by reason of a heating of said resistor and the resultant dispersion of the oxide so formed throughout the fusible salt will not lower the resistance between such metallic members to indicate falsely that said salt is in a fused state.

3. A thermally responsive resistor comprising a pair of spaced metallic conductive members having a gap therebetween at least partially filled with an electrolyte in the form of a salt fusible at a temperature lower than the melting point of said metallic members, said salt normally being in a solid state and nonconductive but being electrically conductive when fused so as to define a conductive path between said members, said salt being in electrical contact with the metal of said metallic members, said metallic members being made solely of metal selected from the group consisting of nickel, aluminum, and magnesium, whose oxides are nonconductive, so that the eiect of continued oxidation of said metallic members by reason of heating of said resistor and the resultant dispersion of the oxides in the fusible salt will not lower the resistance between such members to indicate falsely that said salt is in a fused state.

References Cited in the tile of this patent UNITED STATES PATENTS 2,316,872 Kernen Apr. 20, 1943 2,368,771 Osterheld Feb. 6, 1945 FOREIGN PATENTS 537,155 Great Britain June 11, 1941 

1. A THERMALLY RESPONSIVE RESISTOR COMPRISING AN INNER AND OUTER RELATIVELY SPACED ELECTRODE PAIR, AND DEFINING AN ANNULAR AREA OF SUBSTANTIALLY UNIFORM THICKNESS THEREBETWEEN, THE ANNULAR AREE BEING AT LEAST PARTIALLY FILLED WITH LITHIUM CHLORIDE-POTASSIUM CHLORIDE EUTECTIC, SO AS TO DEFINE A CONDUCTIVE PATH BETWEEN SAID ELECTRODES, SAID RESISTOR ELEMENT BEING CHARACTERIZED IN THAT THE ELECTRODE MEMBERS CONSIST ESSENTIALLY OF A METAL FROM THE GROUP CONSISTING OF ALUMINUM AND MAGNESIUM. 